There's the matter that nothing in UU can take a hit from Hydreigon. Absolutely nothing. The only thing that comes close is Snorlax but he's smoked by Superpower. This is a different scenario than in OU where Hydreigon has several sets and each of them have different counters. One set, the Mixed Attacker set, is unwallable in UU. It's not a matter of it 2HKOing every, it's a matter of it OHKOing almost everything and 2HKOing the rest. If, and only if, those Pokemon get in safely, then yes, Hydreigon will be forced out. But they are all OHKOd on the switch. Are you really going to instantly switch Mienshao in when you see Hydreigon? You get OHKOd by LO Fire Blast.And what if it's a Substitute set? You lose your one offensive check? Unless you're running Scarfed Heracross, Scarf Flygon, AND Mienshao, well then you just dedicated half your team to making sure one Pokemon didn't run wild.
Hydreigon would go BL faster than Chansey did.
Hydreigon would go BL faster than Chansey did.